1. Field of the Invention
The present invention relates to an automatic gain control circuit (AGC) that is used in an automatic loudness control circuit (ALC) for an audio signal or the like.
2. Description of the Prior Art
In an automatic gain control circuit widely used for automatic control of a loudness or automatic control of an output level, provision is made such that an amplification factor of an amplifier can be automatically controlled depending upon an output of the amplifier. More particularly, provision is made such that an output of an amplifier is rectified, the rectified output being converted into a D.C. voltage in a smoothing circuit, and the converted D.C. voltage is adjusted to an appropriate level in a control circuit, followed by controlling the gain of the amplifier by the level adjusted D.C. voltage. The gain of the amplifier is automatically controlled by controlling the input signal level according to the output of the control circuit or by controlling an amount of negative feedback according to the output of the control circuit.
The smoothing circuit is normally formed by an integrating circuit consisting of a resistor and a capacitor. While it is easy in view of a manufacturing technique of resistor elements to precisely control the resistance values of the resistor elements, control for the capacitance of the capacitor is difficult and the capacitance has a large deviation. The deviation of the capacitances results in distribution of time constants of smoothing circuits. In other words, the time required after the rectified output has varied in the direction of increase until the D.C. output of the smoothing circuit is stabilized, and the time required after the rectified output has varied in the direction of decrease until the D.C. output of the smoothing circuit is stabilized, would deviate due to the deviation of the capacitance values. It results in deviation of the attack time after the input signal has increased until the gain of the amplifier is stabilized, and the recovery time after the input signal has decreased until the gain of the amplifier is stabilized.
If the attack time and the recovery time of the automatic gain control circuit should deviate, for instance, in a stereophonic reproducing system, balancing between the left-channel and right-channel sounds would be lost and hence, real live sound could not be reproduced. In addition, even in a monophonic audio system, sound quality of the output would vary among different sets, and in the worst case, distortion of the output would become remarkable.